1. Field of the Invention
This invention relates to a magneto-resistive effect sensor having its magnetically sensitive area formed of a material exhibiting the magneto-resistive effect and to a position detection device employing this magneto-resistive effect sensor.
2. Description of the Related Art
There has so far been known a magnetic position detection device for detecting the rotational position of an rotating object or the position of an object performing a linear movement.
FIG. 1 shows an example of this type of the magnetic position detection device.
A position detection device 100 has an elongated magnetic scale 110, and a magneto-resistive effect sensor (MR sensor) 120, having its magnetically sensitive portion formed by a thin film. One of the magnetic scale 110 or the MR sensor 120 is mounted on a moving object, with the other being mounted on a reference unit.
On the magnetic scale 110, alternate N and S poles are formed along its length as periodic position signals at a recording pitch .lambda..
The MR sensor 120 is held on, for example, a holding mechanism, not shown, and is arranged facing a magnetized surface of the magnetic scale 110 carrying the position signals of the magnetic scale 110. This MR sensor 120 is moved in translation along the position signals of the magnetic scale 110 as it is kept at a pre-set gap distance from the magnetized surface of the magnetic scale 110. The MR sensor 120, thus translated, detects the position signals to convert the detected position signals into electrical signals which are outputted to outside over a flexible cable 130 etc.
With the above-described position detection device 100, the relative position between the magnetic scale 110 and the MR sensor 120 can be detected at an interval P equal to one-half the recording pitch .lambda. to enable detection of the moving position of an object.
Meanwhile, a permanent magnet can be mounted along with an MR sensor for scale signals and an MR sensor for a point-of-origin signals on a head holder in order to apply biasing magnetization across the MR sensor for scale signals and the MR sensor for a point-of-origin signals.
The MR sensor 120 will be explained in further detail.
The MR sensor 120 is comprised of a substrate 121 of a non-magnetic material, such as glass, and a strip-shaped magnetically sensitive portion 122 formed thereon by depositing a ferromagnetic material, such as Fe--Ni or Ni--Co, as shown in FIG. 2. This magnetically sensitive portion 122 demonstrates a magneto-resistive effect in which, when the dc current flows therethrough longitudinally, its resistance becomes maximum and minimum for the minimum strength of the signal magnetic field impressed in a direction perpendicular to the current flowing through the magnetically sensitive portion 122 and which is parallel to the film surface and for the maximum strength of the signal magnetic field impressed in a direction perpendicular to the current flowing through the magnetically sensitive portion 122 and which is parallel to the film surface, respectively.
On this MR sensor 120, there are formed first to fourth magnetically sensitive portions 122a to 122d in a direction parallel to its longitudinal direction as the magnetically sensitive portion 122. The first and second magnetically sensitive portions 122a, 122b are arranged at an interval therebetween equal to a detection pitch P for the position signals of the magnetic scale 110. Similarly, the third and fourth magnetically sensitive portions 122c, 122d are also arranged at an interval therebetween equal to the detection pitch P for the position signals of the magnetic scale 110. The interval between the second and third magnetically sensitive portions 122b and 122c is set to P/2.
The magnetically sensitive portions 122a, 122b are electrically connected in series with each other by an electrode 123a, while the magnetically sensitive portions 122c, 122d are electrically connected in series with each other by an electrode 123b. The end of the magnetically sensitive portion 122b not connected to the electrode 123a is connected by an electrode 124 in series with the end of the magnetically sensitive portion 122c not connected to the electrode 123b. The end of the magnetically sensitive portion 122a not connected to the electrode 123a is grounded via electrode 125a, while the end of the magnetically sensitive portion 122d not connected to the electrode 123b is connected via an electrode 125b to a constant voltage source. By interconnecting the magnetically sensitive portions 122a to 122d in this manner, an equivalent circuit as shown in FIG. 3 is constituted in the MR sensor 120 to permit a sensor output to be detected at the electrode 124.
The operation of the MR sensor 120 is hereinafter explained.
The above-described MR sensor 120 is moved relative to the position signals on the magnetic scale 110 responsive to the object movement. If, for example, the magnetically sensitive portions 122a, 122b of the MR sensor 120 are moved to over the N and S poles of the position signals, as shown in FIG. 4, the magnetically sensitive portions 122a, 122b exhibit a maximum resistance value because the strength of the magnetic field of stray magnetic flux component in the plane of the magnetically sensitive surface is zero. At this time, the magnetically sensitive portions 122c, 122d exhibit the minimum resistance value because the maximum magnetic field of stray magnetic flux component in the plane of the magnetically sensitive surface is applied. The result is that a maximum potential is produced at the electrode 124.
If conversely the magnetically sensitive portions 122c, 122d of the MR sensor 120 are moved over the N and S poles of the position signals, as shown in FIG. 5, the magnetically sensitive portions 122c, 122d exhibit a maximum resistance value because the strength of the magnetic field of stray magnetic flux component in the plane of the magnetically sensitive surface is zero. At this time, the magnetically sensitive portions 122a, 122b exhibit the minimum resistance value because the maximum magnetic field of the component in the plane of the magnetically sensitive surface is applied. The result is that minimum potential is produced at the electrode 124.
It is thus possible with the MR sensor 120 to output at the electrode 124 a signal generated in conformity to a period equal to one-half the recording pitch .lambda. of the position signals, by movement of the MR sensor 120 on the magnetic scale 110, to detect the position of movement of an object.
In the magnetic position detection device 100, the MR sensor 120 and the magnetic scale 110 are adapted to perform relative movement with a pre-set spatial gap therebetween because in general the MR sensor 120 and the magnetic scale 110 cannot be brought in use into contact with each other. The gap length between the MR sensor 120 or the magnetic scale 110 affects the output sensitivity of the MR sensor 120, in much the same way as the recording pitch .lambda. of the position signals of the magnetic scale 110 or the strength of the magnetic field applied from the position signal to the MR sensor 120.
FIGS. 6C shows output characteristics of the MR sensor 120 with respect to changes in the gap length x between the MR sensor 120 and the magnetic scale 110. It is noted that these output characteristics are derived from resistance changes in the MR sensor 120.
The output characteristics shown here are those for a case in which the MR sensor 120 is provided facing a surface of a flat-plate-shaped magnetic scale 110 having position signals recorded thereon, as shown in FIG. 6A, and in which the width L of the recording signals of the magnetic scale 110 is sufficiently longer than the length l along the longitudinal direction of the magnetically sensitive portion 122 of the MR sensor 120, as shown in FIG. 6B. That is, the characteristics shown here are those obtained when the signal magnetic field of the same intensity is applied across the entire area of the magnetically sensitive portion 122 of the MR sensor 120.
The output characteristics of the MR sensor 120 shown here are those in which a peak output value can be detected at a pre-set gap length x0, as shown in FIG. 6c. Thus, with the position detection device 100, an optimum detection output can be obtained by setting the gap length between the MR sensor 120 and the magnetic scale 110 at the time of relative movement therebetween so as to be close to this gap length x0.
However, there are cases wherein the magnetic scale 110 cannot be designed in the flat plate shape, but has to be shaped as a round-rod- or polygonally-shaped bar, given the shape of an article under measurement.
FIG. 7c shows output characteristics of the MR sensor 120 with respect to changes in the gap length x between the MR sensor 120 and the magnetic scale 110 in case the magnetic scale 110 is a round bar.
Specifically, FIG. 7c shows the characteristics in case the MR sensor 120 faces the surface of the magnetic scale 110 in the form of a round bar on which are magnetized the position signals of the magnetic scale 110, as shown in FIG. 7A, and in case a width L of the recording signals recorded on the magnetic scale 110, corresponding to the diameter of the magnetic scale 110, is shorter than the length l along the longitudinal direction of the magnetically sensitive portion 122 of the MR sensor 120, as shown in FIG. 7B. That is, FIG. 7c shows characteristics in case signal magnetic fields of different strengths are applied along the length of the magnetically sensitive portion 122 of the MR sensor 120. The gap length x is represented as the shortest distance between the magnetic scale 110 and the MR sensor 120.
In this case, the output characteristics of the MR sensor 120 are such that the peak output value cannot be detected, as shown in FIG. 7c, with the obtainable output of the sensor being of the order of 50 to 60% of the output obtained with the flat-plate-shaped magnetic scale.
That is, if the MR sensor is in the form of a flat plate, while the magnetic scale is arcuately-shaped, polygonally shaped or formed as a round bar, it is difficult to realize optimum output characteristics. The reason is that, with the MR sensor 120, it is not possible to efficiently detect the stray magnetic flux from the magnetic scale 110 on which is magnetized the position information.
FIG. 8 shows the relative position between the round-bar-shaped magnetic scale 110 and the magnetically sensitive portion 122 of the MR sensor 120.
It is now assumed that the radius r of the round-bar-shaped magnetic scale 110 is 1 mm, the length l along the longitudinal direction of the magnetically sensitive portion 122 of the MR sensor 120 is 2 mm and the shortest gap length x1 between the magnetically sensitive portion 122 and the magnetic scale 110 (the gap length at a longitudinally center position Q1 of the magnetically sensitive portion 122) is 120 .mu.m.
In this case, a gap length x2 at a position Q2 spaced 0.5 mm from the center of the magnetically sensitive portion 122 and a gap length x3 at a position Q3 spaced 1 mm from the center of the magnetically sensitive portion 122, are found as follows: EQU r=l/2=1 EQU x1=0.12 EQU X.sub.2 =-r+(r+X.sub.1 +L ).sup.2 +L +(l/4+L ).sup.2 +L =0.266 EQU X.sub.3 =-r+(r+X.sub.1 +L ).sup.2 +L +(l/2+L ).sup.2 +L =0.501 (1)
It is seen from this that the gap length x2 of the position Q2 is longer than the gap length x1 at the center position Q1 of the magnetically sensitive portion 122 and that, if the strength of the magnetic field generated by the magnetic scale 110 is uniform, the amount of the stray magnetic flux reaching the position Q2 is smaller than the stray magnetic flux reaching the position Q1. Thus, the rate of change of resistance at the position 2 is of the order only of 10% of that at the position Q1. Moreover, from the position Q2 on, there scarcely occurs the change in resistance.
Thus, with the use of the round-bar-shaped magnetic scale 110, it is only a portion near its center that substantially undergoes the magneto-resistive effect, while its end portion demonstrates the function only of a resistor, thus worsening the efficiency.
It is proposed in, for example, Japanese Laying-Open Patent H-8-285509, to set the longitudinal length of the magnetically sensitive portion 122 so as to be shorter than the diameter of the magnetic scale 110 to improve output characteristics of the MR sensor 120 in case of using the round-bar-shaped magnetic scale 110. However, although a sufficient output can be developed if the center position of the magnetically sensitive portion 122 completely coincides with the center of the magnetic scale 110, the output is lowered significantly if the relative position is shifted even to the slightest extent. In this case, it is difficult to realize a stable output. Moreover, the resistance of the device is lowered in an undesirable manner as an electrical device, to render assembling of the magnetic scale device difficult.